GROWTH OF ROOT AND STEM 



21 



that the elongating part of the root occupies a certain area back 

 of the tip. This elongation is due to the elongation of individual 

 cells which are constantly being formed in the growing point of 

 the tip, and are left behind. These cells elongate slowly at first, 

 then rapidly and then slowly again until they cease to elongate. 

 All of the cells in one cross area of the root grow at about the same 

 rate at the same time. Their united action then is manifest in 

 the slow elongation of the root just back of the tip, its more rapid 

 elongation further back, followed by the slow elongation again 

 until elongation finally ceases. 



33. Region of elongation in stems. The region of elonga- 

 tion in stems is determined in a similar way. It occurs just back 

 of the growing point, but covers usually a greater area than in the 

 root. 



34. The motor zone in roots, or region of curvature. 

 After a seedling or a plant has been growing in one direction for 

 a time, if its position be changed so that the root and stem are in 

 a horizontal position, or 



at any angle from the up- 

 right, the root and stem 

 will curve so that the root 

 grows downward and the 

 stem upward. The region 

 of curvature of the root 

 under these circumstances 

 corresponds with the re- 

 gion of elongation. This 

 curvature is made possible 

 because the cells in the 

 region of the root are all 

 elongating. Those on the upper side of the root elongate more 

 than those on the underside and bring about the curvature. 



35. The perceptive zone in roots. The perceptive zone in 

 roots is that part of the root which receives the stimulus causing 

 it to turn downward. It is, therefore, a sense organ. 



The stimulus which causes the root to turn downward is initiated 



Fig. 28. 



Bean seedling placed horizontally and marked to 

 show where the root bends. 



